1. Field of the Invention
This invention relates to an optical disk for optically recording/reproducing data.
2. Description of Related Art
A transparent plastic substrate is suitable for an optical disk from the viewpoints of the convenience of duplication, and particularly, a polycarbonate substrate is low cost.
However, when such a plastic substrate that has relatively large moisture permeability as the polycarbonate substrate is used, only the surface other than a recording surface of the substrate absorbs moisture, causing warp of the substrate. The plastic substrate of the above-mentioned type generates a large warp, especially under the circumstances having a large change of the temperature and humidity. In consequence, an actuator becomes unable to stably follow a focus servo or a tracking servo, resulting in faulty operation or errors.
In consideration of the above warp, a different type of an optical disk has been developed, wherein a moisture-proof film is formed on the surface opposite to the recording surface of the substrate. FIG. 1 shows a cross section of this type of the optical disk disclosed in Japanese Patent Laid-Open No. 1-292639 (1989), in which reference numerals 11, 3, 51 and 9 represent respectively a plastic substrate, a recording film, an ultraviolet light setting resin and a moisture-proof film.
The moisture-proof film 9 may be organic or inorganic. The organic film 9 is formed through application and the inorganic film 9 is formed through sputtering, vacuum evaporation, CVD, etc.
The moisture-proof film, when it is formed on a part of the disk, is effective to prevent the warp of the substrate against a gradual change of the temperature and humidity, but is impossible to avoid the warp generated when the temperature and humidity change abruptly. The reason is that the moisture distribution within the substrate is turned temporarily not to be uniform when the moisture is absorbed abruptly from the part of the substrate where the moisture-proof film is not formed, and the disk is locally expanded.
Therefore, it is necessary to form the transparent moisture-proof film all over the surface of the disk so that the substrate material is not in touch with the air. As is disclosed in the aforementioned prior art No. 1-292639, however, it has been known to be considerably difficult to form the moisture-proof film uniformly all over the surface of the disk. In other words, the mask used in the conventional manufacturing process through spin coating, sputtering, evaporation or the like is inevitably brought into touch with the substrate, and therefore, the film may not be formed all over the surface of the disk and a boundary is generated in the film, or very small holes (voids, pin holes) are generated in the film. As a result, the moisture-proof effect cannot be expected so much. What's worse, the moisture-proof film may peel or cause deterioration.
Further, since the laser beams enter the optical disk from the substrate side, the laser beams also pass through the moisture-proof film formed on the substrate. Accordingly, it is required to form the moisture-proof film without lowering the enhancement effect of the magneto-optical disk, the intensity of the incident light beams which is an essential property for the optical disk, or improvement in decreasing noises. Therefore, the optical constant, thickness, surface roughness of the moisture-proof film should be controlled properly, a highly sophisticated technique is necessary according to the conventional film forming method and the manufacturing cost increases.
Dusts adhered to the optical disk, or wiping the dusts off the optical disk alike causes a damage to the disk. To prevent this, it has been known to form a hard coat film or an antistatic film on the surface of the substrate. However, the original function as a hard coat material is reduced by half if the antistatic function is added. For example, in the case of the polycarbonate substrate which originally has the pencil hardness 3 H as the substrate and 7 H after applied with the hard coat film, the pencil hardness is 4 H if the antistatic function is added to the hard coat film. It has been impossible to achieve both the hard coat function and the antistatic function by one film hitherto.
Since the conventional hard coat material or antistatic material is an ultraviolet light setting resin or a thermosetting resin, and the method of forming the film is mainly spin coating, it has been difficult to form the film all over exposed the surface of the substrate. The innermost periphery or end face of the substrate remain without the film. If the disk is housed in a cartridge in use, for example, a portion of the disk without the film wears from butting against the protruding parts of the cartridge, abrasion dusts adhere to the disk, and the disk may be damaged when the abrasion dusts are removed.
It is proposed in Japanese Patent Laid-Open No. 63-112632 (1988) to form a silicon dioxide (SiO.sub.2) film on the substrate through Liquid Phase Deposition (LPD). According to this method, the whole disk is dipped into the treatment solution, so that the film can be formed uniformly all over the surface of the disk except where the disk is held by a tool. The film formed by this method displays not only good moisture-proof property, but large surface hardness and high antistatic function as indicated in FIG. 2.
However, if the film is formed thin, the film is not sufficiently dense or island-shaped. Since the film cannot interrupt the moisture to pass through, the moisture-proof efficiency is low, which makes it difficult to prevent the warp of the disk.
In the case where the film is formed on such a substrate of little hardness as the polycarbonate substrate, if the film is thin, the hardness of the substrate itself has a greater influence on the disk hardness than the hardness of the film. Therefore, the hardness of the substrate itself becomes the disk hardness so that the film cannot work as the hard coat material.
The antistatic function is achieved because of the electroconductivity of the film, that is, since the film does not store electric charges and therefore hardly generate static electricity. If the film is thin and formed in island-shape, i.e., having boundaries, the electric charges are easily be stored. Therefore, the electroconductivity of the film decreases considerably, whereby the antistatic function is hardly achieved.
Since in forming the SiO.sub.2 layer through the LPD method, hydrofluoric acid is used as a solution, the metallic recording film is corroded by the solution, so that it is not possible to practice the LPD treatment after the recording film is formed. As such, according to the conventional method, the substrate is first subjected to the LPD treatment to form the SiO.sub.2 layer all over the surface thereof, and then the recording layer is formed. However, if grooves or pre-pits are formed on the substrate, the SiO.sub.2 layer is formed also on the grooves or pre-pits when the SiO.sub.2 layer is thick, thereby undesirably changing the configuration of the grooves or pre-pits. Although a slight correction is possible by adjusting the setting of the stamper for forming the substrate, the adjustment needs a high-level technique, and therefore it is difficult to perfectly correct the change of the configuration of the grooves or pre-pits. Moreover, if the film is so thick as to fill in the grooves of the substrate formed by the stamper which is not adjusted, since the optimum value to set the stamper properly cannot be calculated, the proper grooves are not formed, causing the trouble in reading out signals. Accordingly, the LPD method has been hard to be applied to the optical disk.
Another LPD method to form the SiO.sub.2 film all over the surface of the optical disk is disclosed in Japanese Patent Laid-Open No. 2-18729 (1990). FIG. 3 is a cross section of the optical disk with the SiO.sub.2 film formed thereon by the method disclosed in the above prior art. In FIG. 3, reference numerals 1, 2, 3, 4, 51 and 6 are respectively a plastic substrate, a dielectric film, a recording film, a dielectric protection film, an ultraviolet light setting resin and an SiO.sub.2 layer.
According to this method, the optical recording film is formed on the substrate, on which the protection film to protect the optical recording film from the a hydrofluoric acid solution which is used in the LPD method, and thereafter the SiO.sub.2 layer is formed on the whole surface through the LPD treatment. The protection film formed through sputtering or evaporation is not dense enough, with generating many pin holes. Therefore, almost the whole surface of the recording film is corroded. Moreover, since most of the general coating resins are not readily brought in tight contact with the recording film, the solution may infiltrate from the boundary between the resin and recording film or from the pin holes in the protection film to the recording film, so that the recording film is corroded or cracked. Accordingly, conventionally, for the protection film is used an ultraviolet light setting resin.
However, heat is added to the ultraviolet light setting resin when it is set. This heat may deform or warp the plastic substrate, thereby increasing the double refraction, bringing about cracks of the resin layer or recording film in the LPD treatment.